Dispenser type cathode



Feb 28, 1950 J. M. LAFFERTY DISPENSER TYPE CATHODE Filed Jan. 15, 1948 Fig.2.

Fig.1.

Inventor: James M. Laffertg, 13% 45M His Attorney.

patented Feb. 28, 19 56 T "OFFICE DISPENSER TYPE CATHODE James M. Lafferty, Schenectady, N. Y., assignor to General ElectricCompany, acorporation of New York Application January 15, 1948, Serial hlo. 2.402

4 Claims. (01'. 250-2715) My invention relates to dispenser type cathodes and methods of making the same. i

. Dispenser type cathodes have been manufactured in which a mesh of nickel or similar material is filled with a body of powdered electron emissive materialsuch as barium aluminate. The mesh is heated by conduction current to raise the powdered material to a temperature at which electrons are emitted. For a number of applications, it is desirable to obtain a dispenser type cathode having relatively small cross sectional dimensions and it has been difficult toobtain satisfactory life from the cathode made in accordance with the prior art. Prior art dispenser cathodes have also possessed rather low resistance so that a relatively large heating current is required. In accordance with. an important aspect of'my invention, I provide an improved dispenser type cathode and an improved method of making it which results in a very dense supply of electron emitting material and as a result insures adequate emission for a long period'of use.

It is an object of my invention to provide a new and improved dispenser type cathode.

It is another object of my invention to provide an improved dispenser type cathode whichinsures an adequate supply of electron emitting material over a long periodof use. 7

It is another object of my invention to provide an improved dispenser type cathode and method of making which is particularly applicable to cathode structures of small c'rossse'c tional dimensions.

Further objects and advantages of my invention will .become apparent as the following description proceeds, reference bein had to the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing, Fig. l is an end elevational view of a cathode assembly embodyin my invention; Fig. 2 is a side elevational view of the cathode assembly shown in Fig. 1; Fig. 3 is a sectional View taken along the line 3-3 of Fig. 1; Fig. 4 is a perspective view of the cathode element of the assembly shown in Figs. 1-3; and Fig. 5 is a perspective view of apparatus which may be used to advantage in the manufacture of cathodes in accordance with my invention.

Referring now to the drawing, I have shown my invention as applied to a cathode assembly of a type which is particularly adapted for use in electron beam devices such as electron accelerators. As illustrated in the drawing,.the cathode proper comprises a helical wire coil l which may be wound on a suitable mandrel and 2 have any desired cross section. As illustrated in Figs. 3 and 4, the cross section of the illustrated embodiment is rectangular. One side of therec tangle is curved inwardly or indented to pro; vide focusing of the electron beam emitted from the cathode surface. vThe v coil I is supported within ,a channel-shaped shield and focusing member 3 by means of an intermediate channelshaped member including a central member -t of molybdenum or similar material which isco'ateed with a layer of alumina'5. As illustratedthe side walls of the channel member 3 extend be yond the curved surface of the cathode and adapt the shield for its focusing function. The cathode l and shield 3 may be supported in an electron discharge deviceysuch as an electronic ac celerator by the conductors 6 and l which may also provide means for supplying suitable potential to the shield 3. The ends 8 and .9 of the helical coilare brought out through ope ings in the channel 3 and are connected to suitable conductors I0 and II which are likewise sealed through the envelope of the device in which the cathode is to be used. The ends of the coil I. are closed by sheet metal members l2 which are secured"in.:position by welding or. ,by the ,wires I 3 which also bind the conductors '8. and

Sand in and II together. The cathode I and focusing shield 3 are positioned. within alarg er channel-shaped electrode l4 which'extends be; yond the innershield 3 andma'ybeutilized as an, accelerating electrode. Suitable conductors l5 and iii are bonded to the electrode at for supporting it within the device in which it is to be used in insulated relation with respect to the cathode assembly and permitting suitable positive voltage to be applied to it when it is to be used as an accelerating electrode.

In accordance with an important aspect of the present invention, the cavity provided by the helical wire coil I is filled with a dense body of electron emittingmaterial designated by the numeral l1. While a number of materials may be used. such as barium hydroxide, thorium aluminate, or strontium aluminate, I have found that barium aluminate is particularly desirable. The barium aluminate in granular form may be loaded into the interior of the coil and the coil heated to a temperature of approximately 1650 C. (preferably in an atmosphere of hydrogen) to melt the barium aluminate. The barium aluminate forms a solid dense mass which only partially fill-s the interior of the coil l and the process is repeated until the coil is completely filled with a dense fused mass of solid barium aluminate. In'the completed cathode, the barium aluminate extends between adjacent turns of the coil sufficiently to insulate them with respect to one another and renders it possible for the coil l to function as a heater for the cathode. The coil offers appreciably higher resistance than the mesh employed in the dispenser cathodes of the prior art and the increased resistance reduces the heating current requirements of'the cathode.

The method described above which involves alternately filling the coil with barium aluminate powder and melting it down works very satisfactorily and produces a cathode having a very dense solid core. I have foundthat it is possible to produce a cathode having essentially't'he same characteristics by melting a quantity of the barium aluminate and immersing the 'c'dilin the melted material. In Fig. 5, I have illustrated an electrically heated receptacle or boat which may be used for this purpose. As illustrated, the boat is shaped generally the same as the heater "of the cathode to be filled with liquid barium aluminate and is formed of a suitable high tempera- 'ture metal, such as molybdenum. The boat is provided with an electrical heating "coil l9 which may be provided for melting the material. The cathode is immersed in therec'eptacle I8 which is substantially filled with melted'barium aluminate. This is done preferably in an atmosphere of hydrogen or other reducing atmosphere. The heater and core are removed from theboat 'and after the assembly has cooled sufiiciently "the emitting material forms a solid dense core.

The cathode of my invention when employing barium aluminate is operated at temperatures of 1200-1350" C. which is 'sufiicient'to provide for the gradual release of barium from the supply .p'rovided by the dense core.

As an example of a cathode ma'de in accordancewith my invention, I hav'efound that a coil of wire formed of 35 turns of $015 inchdiam'eter "wire provides a cathode approximately /16 "of "an inch *long. This means that there is a very slight space between each turn which is filled with the solidified emissive material "of "the core. The dimension of the square 'coilmaybe, forexample, 4/10 of an inch. A cathode having-these dimensions and manufactured inaccord'ancewith my invention is capable of delivering acurren't of 500 milliamperes, and it has been found to have a useful life exceeding 10,00'0hours. In the "foregoin specification, twomethods of 'pr'oduc ing the solidified core within the coil have been described. In both cases, the resultant mass may be considered as fused or cast and in the appended claims, the word fused is used to denote that the powdered material has been at least partially melted, that is, bonded together in a dense mass.

While I have. shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention in its broader aspects, and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A cathode comprising a helical wire coll providing a heater and a cast body of electron emitting material within said coil and extending between adjacent turns of said coil 'tofhold said turns in insulated relation.

2. A cathode comprising a helical coil of molybdenum wire and 'a'cast mass of barium 'aluminate within said coil and extending'between adjacent turns of said coil to hold said turns "in insulated relation.

3. The method of making a'disp'enser typecathode which comprises forming a hollow coil :and filling the coil with melted barium aluminate.

4. The method of making a cathode including a hollow supportin coil which comprises the steps of loading the coil witha quantity of emitting material in granular form, melting the material to form a dense mass and repeating the loading and melting steps until the "coil is filled with a dense cast-bodyof emitting material.

JAMES M. LAFFERTY.

REFERENCES CITED The following references are of record in the file of this "patent:

UNITED STATES PATENTS Number Name Date 1,813,704 Lederer -July v'7, 1931 1,817,636 Meyer et a1 1- Aug. 4,1931 1,946,603 Von Wede'l Feb. 13, 1 934 2,007,926 Braselton July 9, 1935 2,007,932 Ruben July 9, 1935 2,107,945 Hull et a1. 'Feb. 8, 1938 2,175,345 'Gaidies et al. Oct. 10, 1939 

